Sydney Jekyll is a 2010-introduced and book-only character. She is the mother of Jackson Jekyll in the books. She does however not exist in the webisode cartoon series, TV specials, character profile bio fiction, or anywhere else in the Monster High franchise. She is the Dr. Jekyll who is Jackson's "Dr. Jekyll parent" from Strange Case of Dr. Jekyll and Mr. Hyde who was Henry Jekyll and Edward Hyde. She is the descendant and daughter of the original Dr. Jekyll and Mr. Hyde character. Sydney is a loving mother is is very protective of her little boy, Jackson, but also cares for D.J. Hyde, as well. She wanted Jackson to keep his identity as being a shiftshaper from Jackson to Hyde a secret from the world.
Sydney knew about D.J. all along, but never told Jackson about him, so Jackson didn't know until he was sixteen years old of age. She is a caring and overprotective mother. Sydney is a resident of Salem, Oregon. She is a concerning mother who loves her little boy very dearly. She only exists in the Monster High book series.
Sydney teaches Science Class at school in Merston High. At Merston High, she is a teacher who goes by Ms. J and talks about monsters in school, to which Brett Redding gets very excited about. She teaches them about a couple things here and there. Later, she is telling Jackson about his monster heritage out of his half-human-half-monster heritage of his Hyde side, and begs him to keep it a secret. Sydney is later confronted by Melody Carver outside her house and Melody asks to come in while she is dressed with a swat bun in her hair and she invites Melody in. They talk about Jackson's Dr.-Jekyll-Mr.-Hyde heritage, and Melody promises to keep the Jekyll-Hyde heritage a secret. She gets worried over Jackson being exploited now that press knows his heritage of being a descendant of Dr. Jekyll and Mr. Hyde.